Page 78 - Well Logging and Formation Evaluation

68 Well Logging and Formation Evaluation

as with Archie but are derived in a different way from core data. Tem-
perature determines the excess conductivity (in ohms) resulting for the
clay per unit Q v (in cc).
An equation that may be used to relate B to formation temperature and
water resistivity, R w, as published by Thomas, is as follows:

1 23
2
.
)
+
.
.
.
B =- ( 1 28 0 255*T - 0 0004059* T ) (1 + (0 04*T - 0 27 * R w )
.
.
(5.1.2)
where T is measured in degrees Celsius.
Q v (in meq/unit pore volume in cc) is related to the cation exchange
capacity (CEC) (in meq/100g) of the clay as measured in a laboratory.
Q v may be derived from the CEC using:
Q v = CEC density (100 f ) (5.1.3)
*
*
CEC can be measured by chemical titration of crushed core samples. It
is dependent on the type of clay. Typical values for clay types are shown
in Table 5.1.1.
It has to be said that core-derived measurements based on crushed
samples are probably unrepresentative, since the crushing process will
expose many more cation exchange types than will be available in the for-
mation. Moreover, I have never been able to relate Q v to any log-derived
parameter (e.g., porosity, V sh ) with any success.
A further uncertainty relates to the factor B, which is typically derived
using a “standard” correlation that may or not be applicable. A far better
approach is to derive the combined factor BQ v from logs in a known water-
bearing sand. I will now present a useful method of doing this. First of
all, the assumption is made that BQ v obeys an equation of the form:

BQ v = (f c - ) f C ( * ) f . (5.1.4)

Table 5.1.1
Typical Properties of Clays

CEC Grain Density Hydrogen
Clay (meq/100 g) (g/cc) Index
Kaolinite 3–15 2.64 0.37
Illite 10–40 2.77 0.09
Montmorillonite 80–150 2.62 0.12
Chlorite 1–30 3.0 0.32